1. Field of the Invention
The present invention relates to a controller for an electric motor of an axial air-gap type.
2. Description of the Related Art
The electric motor of the axial air-gap type having a rotor having a permanent magnet, two stators arranged on both sides of the rotor in a rotation axis direction of the rotor, and an armature winding mounted to each stator is formerly known (e.g., see Japanese Patent Laid-Open No. H10 (1998)-271784, Japanese Patent Laid-Open No. 2001-136721, and Japanese Patent Laid-Open No. H6 (1994)-245458). In accordance with such an electric motor of the axial air-gap type, relatively high output torque can be generated while the axial length of the rotor of the electric motor is shortened.
Meanwhile, when an electric motor of an axial air-gap type operates in a low torque and low speed operating state where an output torque and a rotating speed of the rotor are small, a variation in the output torque tends to appear remarkably. Then, when such an electric motor is installed in a hybrid vehicle or an electric vehicle as the driving power source for the vehicle, a car body will vibrate due to a variation in the output torque of the electric motor (so-called torque ripple) when the vehicle starts moving.
Therefore, it is desirable to suppress a variation in the output torque of the electric motor in the low torque and low speed operating state.
However, the techniques disclosed in the above-stated Japanese Patent Laid-Open No. H10 (1998)-271784 and Japanese Patent Laid-Open No. 2001-136721 do not provide a countermeasure for suppressing a variation in the output torque of the electric motor.
In the technique disclosed in Japanese Patent Laid-Open No. H6 (1994)-245458, in order to suppress a variation in the output torque of the electric motor, an installation position of an armature winding to one stator is displaced from an installation position of an armature winding to the other stator in the circumferential direction of the stators (the direction around the axis center of the rotor). Alternatively, a position of a permanent magnet mounted to one face of the rotor in the axis direction is displaced in the circumferential direction of the rotor from a position of a permanent magnet mounted to the other face. According to the technique disclosed in this Japanese Patent Laid-Open No. H6 (1994)-245458, a variation in the output torque can be suppressed irrespective of the operating state of the electric motor.
The technique disclosed in this Japanese Patent Laid-Open No. H6 (1994)-245458, however, has a problem that, as compared with the case where the armature windings are installed at the same position for both stators or where the permanent magnets are mounted at the same position for both faces of the rotor, the maximum torque that the electric motor can output is decreased, or the energy efficiency of the electric motor is decreased.